1. Field of the Invention
The present invention relates generally to devices for the test and measurement market, and more particularly to sensors which have stepped pads for connectivity.
2. Description of the Related Art
Technological advancements have propelled the growth of sensors in the test and measurement space, and manufacturing environments routinely uses sensor-based test and measurement instruments to ensure quality production. The development of high-value resistor kits allows the unhampered use of high-impedance sensors for accurate measurements without interference from external noise, solder-flux residue, particle tracking, bias currents, and distant charges that can make repeatable measurements difficult.
There are a wide variety of devices used in test and measurement. One type of device often used is a sensor, including but not limited to, an accelerometer, pressure sensor, optical sensor and the like. In the test and measurement marketplace, a smaller footprint of the test and measurement device is highly desirable. A smaller size device is less disruptive/intrusive to the unit under test and/or allows more units (uni-axial or tri-axial) to be placed on the article under observation.
As a non-limiting example, a suitable sensor is an accelerometer which behaves as a damped mass on a spring. When the accelerometer experiences an acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration.
In commercial devices, piezoelectric, piezoresistive and capacitive components are commonly used to convert the mechanical motion into an electrical signal. Piezoelectric accelerometers rely on piezoceramics (e.g. lead zirconate titanate) or single crystals (e.g. quartz, tourmaline). Piezoresistive accelerometers are often used in high shock applications. Capacitive accelerometers typically use a silicon micro-machined sensing element. Their performance is superior in the low frequency range and they can be operated in servo mode to achieve high stability and linearity.
Modern accelerometers are often small micro electro-mechanical systems (MEMS) and can consist of little more than a cantilever beam with a proof mass (also known as seismic mass). Damping results from the residual gas sealed in the device. As long as the Q-factor is not too low, damping does not result in a lower sensitivity.
Under the influence of external accelerations the proof mass deflects from its neutral position. This deflection is measured in an analog or digital manner. Most commonly, the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple, reliable, and inexpensive. Integrating piezoresistors in the springs to detect spring deformation, and thus deflection, is a good alternative, although a few more process steps are needed during the fabrication sequence. For very high sensitivities quantum tunneling is also used; this requires a dedicated process making it very expensive. Optical measurement has been demonstrated on laboratory scale.
Another, far less common, type of MEMS-based accelerometer contains a small heater at the bottom of a very small dome, which heats the air inside the dome to cause it to rise. A thermocouple on the dome determines where the heated air reaches the dome and the deflection off the center is a measure of the acceleration applied to the sensor.
Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-of-plane device three axes can be measured. Such a combination may have much lower misalignment error than three discrete models combined after packaging.
Irrespective of the type of device, there is a need for devices in the test and measurement market, particularly for sensors, which have smaller footprints.